As a method of drilling processing of a laminated board or a multi-layer board that is used in a printed wiring board, a method involving conducting hole boring processing by placing, as an entry board, metal foil of aluminum or the like used alone or a sheet obtained by forming a resin composition layer on a surface of the metal foil (hereinafter, the sheet is usually referred to as “entry sheet for drilling”, or also simply referred to as “entry sheet”) on the top of one laminated board or multi-layered board or a plurality of laminated boards or multi-layered boards piled has generally been adopted laminated board. Although commonly a copper clad laminated board is often used as the laminated board, the laminated board may be a “laminated board” that does not have a copper foil on an outer layer. In the present specification, unless stated otherwise, the laminated board refers to a copper-clad laminated board and/or a “laminated board” that does not have a copper foil on an outer layer.
Recently, with the demand for improved reliability of printed wiring boards and the progress made in increasing printed wiring board density, there is a need for high-quality drilling processing of laminated boards or multi-layer boards that, for example, improves hole position accuracy and reduces hole wall roughness. To respond to these needs, a hole boring processing method that uses a sheet of a water-soluble resin such as polyethylene glycol (e.g., refer to Japanese Patent Application Laid-Open No. 4-92494), a lubricant sheet for drilling in which a water-soluble resin layer is formed on a metallic support foil (e.g., refer to Japanese Patent Application Laid-Open No. 5-169400), an entry sheet for drilling in which a water-soluble resin layer is formed on aluminum foil formed with a thermosetting resin thin film (e.g., refer to Japanese Patent Application Laid-Open No. 2003-136485), a lubricant sheet for drilling obtained by blending a non-halogen colorant in a lubrication resin composition (e.g., refer to Japanese Patent Application Laid-Open No. 2004-230470) and the like have been proposed and put to practical use.
Further, with lasting progress toward higher-density printed wiring boards, the recent trend in drilling processing of laminated boards or multi-layer boards has the following characteristics. Namely, firstly, the interval between the drilling processed holes is becoming much narrower due to the increasing density of wiring circuits. Accordingly, to maintain insulating properties between the drilling processed holes, much superior hole position accuracy is required. Secondly, since the diameter of the drilling processed holes is becoming smaller and the strength of the smaller-diameter drill bits that are used lowers, drill bit breakage during drilling processing is becoming a problem. Namely, much better resistance to drill bit breakage is required.
To respond to these needs, it has been proposed to control the number average molecular weight of the polyethylene glycol and polyethylene oxide used as a resin composition for the entry sheet (e.g., refer to International Publication No. WO 2009151107). On the other hand, an attempt has also been made to improve resistance to drill bit breakage by providing a carbon-based coating on the tools (e.g., refer to Japanese Patent No. 4782222).